The present invention relates generally to instrumentation amplifiers, and more particularly to circuitry and techniques for providing accurate instrumentation amplifier operation in both positive and negative portions of a common mode input voltage range.
The closest prior art is believed to be the assignee's INA 168 positive common mode voltage instrumentation amplifier, a simplified diagram of which is shown in FIG. 1. Referring to FIG. 1, prior art instrumentation amplifier 1 has positive common mode voltage operation between a single supply voltage V+ and ground. Amplifier 1 includes an operational amplifier 7A having a (+) input coupled by a conductor 12 to one terminal of a precision input resistor 5, which may, for example, have a resistance Rin equal to 5 kilohms. The other terminal of resistor 5 is coupled by a conductor 2 to receive an input voltage Vin+ and also is connected to one terminal of a “sense” or “shunt” resistor RS, the other terminal of which is connected by a conductor 3 to one terminal of an external load 4, the other terminal of which is connected to ground. Shunt resistor RS has a very low resistance, typically 10 milliohms to 1 ohm, and a current IS flows through shunt resistor RS into load 4 in response to a positive value of Vin+. The current IS develops a small sense voltage across shunt resistor RS. Conductor 3 is connected to one terminal of another precision input resistor 6, which also has the same resistance Rin. The other terminal of resistor 6 is connected to the (−) input of operational amplifier 7A. Resistor 6 is precisely matched to resistor 5.
The output of operational amplifier 7A is connected by a conductor 10A to the base of an NPN transistor Q1, the emitter of which is connected by an output conductor 11A to one terminal of a load resistor RL, the other terminal of which is connected to ground. The collector of transistor Q1 is connected by conductor 12 to provide feedback to the (+) input of operational amplifier 7A. Positive common mode voltage instrumentation amplifier 1 thus precisely measures the voltage across shunt resistor RS and produces an output voltage Vout on output conductor 11A, where Vout=(IS)(RS)(RL)/Rin. Thus, Vout accurately represents the current IS supplied to load 4 by Vin+.
The voltage on either conductor 2 or conductor 3 of FIG. 1 can be considered to be the positive common mode voltage. The supply voltage V+ typically has a value in the range from a few volts up to approximately +18 volts. Vin+ can be any value from roughly 1.4 volts to roughly 60 or more volts. The feedback provided by transistor Q1 keeps the voltage difference between the (+) and (−) inputs of operational amplifier 7A at nearly zero, and the current Iin2 through resistor 6 and hence the resulting voltage drop across it, are essentially zero. Therefore, the voltage across resistor RS is equal to the voltage developed across 5 kilohm resistor 5. This voltage across resistor 5 produces a current Iin1 that flows through transistor Q1 and resistor RL.
The common mode voltage on conductor 3 must be positive, because amplifier 7A will turn transistor Q1 off, causing positive common mode voltage instrumentation amplifier 1 to be inoperative if Vin+ is below ground.
There are various applications of positive common mode voltage instrumentation amplifier 1 of prior art FIG. 1, including automotive applications in which both positive and negative transient voltages may occur. However, in such applications, positive common mode voltage instrumentation amplifier 1 may produce undesirable “glitches” in the output voltage Vout.
Thus, there is an unmet need for an instrumentation amplifier which has a wide common mode input voltage range in order to effectively reject a substantial range of both positive and negative transient input voltage spikes.
There also is an unmet need for an instrumentation amplifier which is operative through a wide common mode input voltage range including large-magnitude negative common mode input voltage values substantially lower than the negative supply voltage, and also including large-magnitude positive common mode input voltage values substantially greater than the positive supply voltage.